Final Fantasy VI is my favorite of the series, the atmosphere is just perfect, mature story, great twist, great cast, and on and on.

Least favorite has to be XI, FF should have stayed out of the MMO game.

Overall Order (excluding XIII which I have not played)
VI > VII > VIII > IX > IV > X > V > II > XII > III > I > XI

In order and I don't feel like giving an explanation.

1.) VIII

2.) X

3.) XIII

4.) VII

5.) XII

6.) IX

7.) VI

8.) IV

9.) V

10.) III

11.) II

12.) I

I will not put XI in it because I can compare a single player game to an MMO. But it is of the better Final Fantasys and is my favorite MMO.

Best=ff7. one of my favorite games of all time. the characters were memorable, the story was great and I liked the flow of the game

Worst= toss up between ff12 and ff13, could never get into either one, to me they were the dying blows from that franchise as I will never buy another FF again.

Best = FFX

The characters were great and deep, the history was the best, presentation, art, music, voices, battle system everything was so new and amazing, the game was very long with plenty of side quest and mini games.

Worst = FFXIII

NO TOWNS, terrible characters, main history ends in 25 hours, history was complete garbage and you never knew what were the characters supose to do, the CGI scenes were terrible there was so much things happening at once that you could no concentrate in something, music sucked and the eidolons were useless

My favorite is FF6, worst is FF 12.

Full ranking: 6, 10, 13, 7, 4,2,3,1, 12

Not listed are 5, 8,, and 9 because I haven't played them enough.  Also 11 because lol

I've finished all main Final Fantasy titles except XIII, which I was very excited about at one point but then I saw a friend playing it and got his impressions then I decided I would not buy it and would play it someday, but I'm still making my mind up about that 'cause I know it's long and while I'm not expecting much I'm sure it'll be a hassle once I get to it.

My favorite Final Fantasy is VII. It was just revolutionary for its time and I had the pleasure of playing the game as soon as it released on PlayStation. It was the most expensive game for its day and it showed. The cinematic direction was just unprecedented, the characters and story was vintage Final Fantasy and I loved the steampunk setting. The materia system to this day is the best of the series. It was great to collect them, and it was even more fun to combine them and get awesome effects in battle. The story was the most dramatic and developed at that point, and that was thanks at large to the CD format which allowed no constraints for text dialogues, and the unforgettable FMV sequences on which Square, still very fresh-new on 3D development, made innovative strives integrating gameplay action on top of CG videos.

The story was more profound than any other Final Fantasy to the date, the characters were great and all of them had backstories to uncover. The were great extras like Zack's story which inspired a whole new game (Crisis Core). FFVII also has The Best FF villain.  Period. No argument there. The story had awesome twists, great fights and dramatic moments worthy of the Final Fantasy name. Lots of good side-quests, very varied gameplay, chocobo breeding, vehicles, motorcycling, amazing summon animations! One of the best if not the best FF music soundtrack. Final Fantasy VII is at least on the top 5 games in history!

Worst Final Fantasy? Well, I haven't played XIII, but my belief is there is no "bad" Final Fantasy game. They are all, at least, good games. I would have to say I didn't enjoy Final Fantasy II (NES translated title) that much. For me it didn't add much of anything new to the series and the changes they made kinda lost the direction the first game had. It felt like Final Fantasy was already having an identity crisis still in its infancy. I'm glad they returned to form for the next NES game and stayed very true to its form until Final Fantasy VIII happened and after that it's been up and down but they have never recovered the splendor of the best of the first 7 titles.

My ranking:

7, 6, 10, 5, 4, 12, 9, 1, 3, 8,  2

lol our top 5 FF's are the same

{adds to friends list}

Best... I'd say 4.

Though 6 was my favoirte.

4 had the most consistant storytelling.

Worst... I'd go 7.  Worst story, only one in the series that's hard to look at.

Heck even mystic quest has it's purpose.

I thought it was you for a second darth haha

The best for me is Final Fantasy VI.  I could list a million reasons why, but people who have played the game already know, and those who haven't have already read tons of reasons why.  There is a reason it ranks consistently as the top Final Fantasy and is hailed by many as the best RPG of all time.  It's my favorite game of all time and it's just great.

My least favorite would probably have to be XII.  The game didn't move me or suck me in at all and I found the story boring.  I admittedly didn't finish it, but I put in a good 40 hours into before sticking it on my shelf to collect dust for the rest of it's days.

I also haven't played II, III, XI, or XIII  Although I'm not sure if XI should even be included in this discussion, as it's a completely different type of game.

My ranking would probably be something like:

VI, IV, IX, V, VII, I, VIII, X, XII